1. Field of the Invention
The present invention relates to a constant-voltage generating device, such as a current-controlled voltage generating circuit, for generating an intermediate potential for maintaining a constant current flowing through a differential amplifier.
2. Description of the Related Art
In recent years, a MOS type semiconduct or memory, in which MOS transistors are integrated, includes not only digital circuits but also a number of analog circuits. For example, in a dynamic RAM (DRAM), a voltage down converter employed to assure the reliability of the MOSFET is a typical analog circuit. The voltage down converter step-downs an external voltage V.sub.cc in a memory chip, generates a voltage V.sub.int lower than the external supply voltage and uses the voltage V.sub.int as a source voltage of the memory chip.
Such an analog circuit mounted on a chip generally incorporates a voltage comparator circuit of differential amplifier type in order to compare a reference voltage V.sub.ref with a generated voltage V.sub.int. The voltage comparator circuit of differential amplifier type inevitably consumes a DC through current because of its structure. Therefore, it is very important in designing of a voltage drop circuit to maintain a satisfactory response characteristic with a low through current.
FIG. 1 shows a relationship between the current consumption and the response characteristic in a voltage down converter on the basis of circuit simulation. In FIG. 1, the relationship between the current consumption and the response characteristic of a differential amplifier DA having a constant-current controlling circuit is compared with that of a differential amplifier DA which does not have a constant-current controlling circuit. A response characteristic curve AL indicates the characteristic of the differential amplifier DA having a non-current controlling circuit A and a response characteristic curve BL indicates the characteristic of the differential amplifier DA having a constant-current controlling circuit B. The abscissa represents current consumption under the conditions of a voltage of V.sub.cc +10% at a low temperature, in which the current consumption is maximum, and the ordinate represents response characteristic under the conditions of a voltage of V.sub.cc -10% at a high temperature, in which the response characteristic is maximum.
In the differential amplifier DA, the greater the current consumption, the shorter the response time, whether the constant-current controlling circuit is present or not. However, the differential amplifier DA having the constant-current controlling circuit B is more advantageous to assure an operation even under the worst conditions, in consideration of variance of parameters. For example, if the response characteristic is normalized at the current value of a point C in the differential amplifier DA having the constant-current controlling circuit B shown in FIG. 1, a response time of the differential amplifier DA having the non-constant current controlling circuit A is 3.7 times that of the differential amplifier DA having the constant-current controlling circuit B. On the other hand, to obtain a satisfactory response characteristic, the current consumption of the differential amplifier DA having the non-constant current controlling circuit A is three times that of the differential amplifier DA having the constant-current controlling circuit B. Since the constant-current control is also effective to stabilize a DC output level of a differential amplifier and assure a gain, various constant-current controlling circuits are incorporated in a memory device.
The constant-current controlling circuit has been positively employed not only in the voltage down converter but also in various voltage generating circuits (an intermediate potential generating circuit or a booster circuit) mounted on a chip, or in a system comprising a small amplitude data transmission system using a differential amplifier (e.g., JSSC, Vol. 26, No. 11, Nov. 1991, pp 1498-1505). A typical constant-current controlling circuit has a structure in which differential amplifiers integrated in a chip respectively incorporate current-controlling MOS transistors. In this structure, an intermediate potential, sufficient to cause a current-controlling MOS transistor to operate in a pentode region (saturated region), is input to the gate electrode of a current-controlling MOS transistor. A circuit for generating the intermediate potential is called a current-controlled voltage generating circuit, which is generally shared by a plurality of differential amplifiers.
FIG. 2 is a circuit diagram showing a current-controlled voltage generating circuit and a circuit system for controlling constant current. The current-controlled voltage generating circuit includes a reference voltage generating circuit 1A, a conventional continuous control type constant-current circuit 2 and a load transistor Q.sub.2. The reference voltage generating circuit 1A generates a reference voltage V.sub.r. The continuous control type constant-current circuit 2 includes a differential amplifier DA using the reference voltage V.sub.r as a reference potential, a current controlling transistor Q.sub.1 having a gate electrode controlled by an output signal from the differential amplifier DA, and a standard resistor R.sub.c serially connected between the transistor Q.sub.1 and the power source. The load transistor Q.sub.2 converts a current value to a voltage value. The transistor Q.sub.2 and a transistor Q.sub.3 constitute a current mirror.
An output voltage V.sub.cm from the current-controlled voltage generating circuit is input to the gate electrode of the current controlling transistor Q.sub.3 of each of differential amplifiers 3, thereby performing constant-current control. An operation principle of the current-controlled voltage generating circuit having the aforementioned structure will be described in brief. A reference current I.sub.1 caused to flow through the load transistor Q.sub.2 by the continuous control type constant-current circuit 2 is expressed by the following equation: EQU I.sub.1 =V.sub.r /R.sub.c . . . (1)
Since the transistors Q.sub.2 and Q.sub.3 constitute a current mirror, if the lengths of the gate electrodes of the transistors Q.sub.2 and Q.sub.3 are the same, a current I.sub.2 flowing through each differential amplifier is expressed by the following equation: EQU I.sub.2 =(W.sub.2 /W.sub.1).times.I.sub.1 . . . (2)
where W.sub.1 and W.sub.2 are the widths of the gate electrodes of the transistors Q.sub.2 and Q.sub.3, respectively.
As clear from the above equations (1) and (2), I.sub.2 is a constant value only depending on the reference voltage V.sub.r and the standard resistor R.sub.c, independent of the supply voltage V.sub.cc, temperature and transistor characteristics. Further, the current I.sub.2 flowing through each of the differential amplifiers can be set to a desired value by suitably setting the device dimensions of the transistors Q.sub.2 and Q.sub.3.
Thus, the current-controlled voltage generating circuit shown in FIG. 2 is a very stable circuit, since the current value is determined only depending on the reference voltage value and the standard resistor value.
However, the minimum supply voltage to operate the current-controlled voltage generating circuit is defined by a current biasing stage in which the standard resistor R.sub.c and the two MOS transistors Q.sub.1 and Q.sub.2 are connected in series. In theory, the minimum supply voltage V.sub.min to operate the current-controlled voltage generating circuit is expressed by the following equation: EQU V.sub.min =V.sub.r +V.sub.t . . . (3)
where V.sub.t is the threshold voltage of the transistor Q.sub.2. For example, if V.sub.r =1.5 V and V.sub.t =0.5 V, V.sub.min =2.0 V.
Actually, however, since the conductance of the current controlling transistor Q.sub.1 is limited, voltage drop occurs between the source and drain of the transistor Q.sub.1.
Hence, a supply voltage of 2.5 V or higher is required to maintain the constant current in the current-controlled voltage generating circuit.
In addition to the above problem of the lowering of the minimum supply voltage V.sub.min due to the voltage drop, the following problems due to the manufacturing process arise. The threshold voltage V.sub.t is different in manufactured chips due to fluctuation in the manufacturing process. In consideration of this matter, the minimum supply voltage V.sub.min must be higher (about 2.8 V). Therefore, the margin of the current-controlled voltage generating circuit to the V.sub.cc of 3.3 V employed in devices of 64 MDRAM generation is considerably reduced.
Moreover, the current-controlled voltage generating circuit defines the operation margin of a low supply voltage side of a DRAM.
The current-controlled voltage generating circuit requires two operations: constant-current control and current-to-voltage conversion. Since the two operations are achieved by the aforementioned single current biasing stage in which the three elements are connected in series, the operating limit voltage of the circuit cannot be low.
It is considerably disadvantageous that the operation limit voltage cannot be low, particularly in the following case: when, in the future, the scaling coefficient of the supply voltage becomes smaller than that of the threshold voltage of a MOS, i.e., if only the supply voltage is lowered to assure the reliability of the device MOSFET, although the threshold voltage V.sub.t cannot be lowered due to sub-threshold characteristics of the transistor. In this case, it is considerably disadvantageous that the operating limit voltage cannot be low.
As described above, the conventional current-controlled voltage generating circuit, having a structure in which a standard resistor, a current controlling transistor and the current-to-voltage converting load transistor connected in series, has a drawback in that when the supply voltage is lowered, the difference between the minimum supply voltage required to stably operate a circuit and the supply voltage supplied to the circuit becomes small, resulting in unstable operation.